In order to get optimum performance from high resolution lithographic lens assembles it is imperative that all lenses in a given assembly be centered, within a few microns, to a common optical axis. Further, these lens mounts are expected to maintain an extremely high level of performance under a multitude of environmental conditions during use, storage and shipping.
Lenses are commonly affixed within lens cells with mechanical devices. Differences in the coefficient of thermal expansion between the lens and the lens cell can lead to decentration and intolerable stresses in the lens during ambient temperature changes.
One approach to the solution of the foregoing problems is given by U.S. Pat. application Ser. No. 818,925 filed Jan. 15, 1984 now U.S. Pat. No. 4,733,945 issued 3/29/88 which is assigned to the same assignee as the present invention. This approach calls for the lens to be mounted to a lens cell through flexure means. These flexure means return the lens to a predetermined spatial relation to the lens cell following temperature excursions away from, then back to, the datum temperature. Following the lens being mounted to the lens cell the outer diameter and upper and lower surfaces of the lens cell are precision machined with respect to the optical axis of the lens.
While this approach offered significant advantages over the prior art it does have several drawbacks. There is a chance of damaging the lens mounted in the lens cell during the machining of the lens cell. Also, if an error were made in machining the lens cell it is relatively difficult to remove the lens for installation in another lens cell. This last feature is desirable in that the lenses are quite expensive.
The present invention eliminates or ameliorates the foregoing disadvantages of the prior art by providing a lens mount unaffected by large temperature excursions, which can be easily and safely machined without endangering the lens and which can be easily disassembled. In addition, the lens mount provided is very stable and has a high resonant frequency.